The present invention relates to silicone polycarbonate block copolymers and polycarbonate blends thereof having reduced haze and to a method for making. More particularly, polycarbonate oligomers having at least one terminal haloformate group are initially formed under interfacial reaction conditions and thereafter reacted with polydiorganosiloxane having terminal hydroxyaryl groups.
Silicone-polycarbonate block copolymers have been recognized for their low temperature ductility and flame retardance. These block copolymers can be made by introducing phosgene under interfacial reaction conditions into a mixture of a dihydric phenol, such as bisphenol A (BPA), and an hydroxyaryl terminated polydiorganosiloxane. The polymerization of the reactants can be facilitated by use of a tertiary amine catalyst. Although the resulting silicone polycarbonate block copolymer has been found to have improved flame retardance compared to polycarbonate, the clarity of the silicone polycarbonate copolymer is often adversely affected.
Experience has shown that if phosgene is introduced into a mixture of dihydric phenol and phenol capped polydiorganosiloxane under interfacial reaction conditions, various reactions can occur, because dihydric phenol and phenol capped polydiorganosiloxane oligomerize at different rates. As a result, several polycarbonate products can be formed including siloxane rich copolymer and polycarbonate substantially free of silicone. This can result in silicone polycarbonate block copolymer which may be extremely hazy or nearly opaque.
It would be desirable therefor to be able to make silicone polycarbonate block copolymer having substantially reduced haze and substantially improved transmittance. It also would be desirable to provide an improved method for making such improved silicone polycarbonate block copolymer.
As used hereinafter with respect to the description of the silicone polycarbonate block copolymer made in accordance with the practice of the invention, the term "haze" means that percentage of transmitted light which in passing through the specimen deviates from the incident beam by forward scattering (ASTM D 1003-61).
As used hereinafter with respect to the description of the silicone polycarbonate block copolymer of the invention, the term "transmittance" means the ratio of transmitted to incident light in accordance with Method E 308 (ASTM D 1003-61).